Patent document 1 discloses a tire position determination system (auto-location function) known in the prior art that automatically determines tire positions to monitor the air pressure of each tire. The system disclosed in patent document 1 includes first sensors (4a to 4d) arranged in wheels (2a to 2d), four second sensors (5a to 5d) corresponding to certain positions of the vehicle, and a measurement system (3) that determines wheel positions. The first sensors transmit signals (S4a to S4d) indicating the wheel positions to the measurement system. The second sensors measure angle positions of the wheels and output the measured values (S5a to S5d). The measurement system determines phase positions (W1a to W3a, W1b to W3b) of the signals from the first sensors based on the measured values and checks whether or not the phase positions are within predetermined tolerable ranges (WTa, WTb) during a predetermined monitor period. This determines the wheel positions.